Surface wave integratable filters (SWIF's) comprise a pair of transducers each generally composed of a pair of separated interleaved comb-shaped electrodes of conductive elements (teeth) coupled to the surface of a piezoelectric medium for producing and receiving acoustic surface waves. Means are provided for impressing an electrical signal across one of the pair of electrodes, referred to as the input transducer, and for recovering signals intercepted by the other of the pair of electrodes, referred to as the output transducer. In a simplified embodiment a piezoelectric ceramic wafer is polarized perpendicularly to the propagating surface and the waves travel at right angles to the electrode elements. In crystalline materials, the waves may travel at an acute angle to the elements, the particular angle in a given case being a function of the crystallography of the material.
The surface waves launched by the transmitting, or input transducer, propagate across the medium and are converted back into an electrical signal by the receiving or output transducer. In principle, the pattern of teeth in the interleaved comb-shaped electrodes is analogous to that of an antenna array. Consequently, exceptional frequency selectivity is possible without large, critical components normally associated with frequency selective circuits. Thus, this device, with its small size, may be of particular use in solid-state integrated circuits requiring a high degree of signal selectivity.
Since the input and output transducers are separated, the acoustic wave requires a finite time to travel from one to the other. At the output transducer, a portion of the acoustic wave energy is converted to electrical energy and delivered to a connected load, a portion continues on past the output transducer, and a portion is relfected back toward the input transducer. This reflected surface wave, which is identical in frequency to the transmitted surface wave, but smaller in magnitude, intercepts the input transducer where it too is reflected back toward the output transducer, resulting in a diminished replica of the original surface wave signal at the output transducer. Unfortunately, the diminshed replica arrives later than the transmitted signal (the time delay being equal to twice the transit time from input to output) and gives rise to objectionable interference.
While SWIF devices are designed to function by the propagation of surface waves, in the transduction process the launching transducer creates bulk waves which travel through the body of the medium. These bulk waves, upon reaching a surface of the medium, most noticeably the surface opposite the transducer bearing surface, are reflected and induce voltages in the receiving transducer.
If, for example, the SWIF is used as the signal-selective device in a television intermediate-frequency amplifier, the reflected IF waves result in "ghosts" or multiple images in the reproduced image making it highly undesirable if not completely unacceptable for normal viewing.
Known methods of overcoming the problem of reflected surface waves include selecting the output load impedance to maximize energy transfer thereto, depositing surface wave attenuating material between the transducers (which attenuates the reflected wave three times whereas the transmitted wave is attenuated only once), and reducing the time delay by placing the output transducer closer to the input transducer.
The adverse effects of bulk waves are effectively minimized in the well-known sidestepping SWIF configuration. Briefly, such devices use a surface wave coupler between transmitting and receiving transducers to guide surface waves in an S-shaped path which the bulk waves of major concern cannot follow. Sidestepping SWIF's, however, require almost twice the surface area as conventional SWIF's, which adds to their cost.
The processes and techniques used to manufacture SWIF's are complex and intricate and a summary at this point may be helpful. A seed of lithium niobate is used to grow an elongate crystal, called a boule, which is then poled by application of electrodes bearing a suitable DC potential. Poling results in imparting piezoelectric properties to the boule. Current practice is to cut the boule into thin wafers such that the Z axis is used for surface wave propagation. One side of each wafer is then ground and polished to a mirror-like finish and thoroughly cleaned.
The polished surface of the wafer, now approximately 2 inches in diameter, is covered with a layer of aluminum or gold approximately 4000A thick by the familiar vacuum deposition process of exposing the wafer to a heated element containing the depositing metal in an evacuated environment. A coating of photoresistive material such as Shipley AZ1350H is uniformly applied to the metal surface and the wafer baked at approximately 75.degree. C for a period sufficient to dry the resist material. The wafer is then exposed by either projection printing or contact printing photolithograhic techniques, using a source of ultraviolet light, to form a latent image of the desired metallization pattern in the resist material. After developing, an etching agent, in this case a mixture of phosphoric acid, acetic acid and nitric acid, is then used to remove the undesired metal film leaving behind the metallized pattern forming transducers and suitable connection pads. SWIF devices are quite small, typically 140 .times. 460 mils and, therefore, a single 2 inch diameter wafer is generally simultaneously imprinted with 30 to 40 individual SWIF device patterns, which are later separated by either laser cutting or sawing the wafer. The devices, once separated, are placed within a suitable housing, mounted and connected to leads as, for example, described in application Ser. No. 366,811 filed June 4, 1973 in the name of Guy N. Falco entitled "CERAMIC PACKAGE AND METHOD FOR A SURFACE WAVE SELECTIVE DEVICE", and now U.S. Pat. No. 3,872,331, and assigned to the assignee of the present application.
Techniques of manufacturing and testing vary between manufacturers, some preferring to test the individual devices on the wafer, using conventional solid-state manufacturing techniques while others test after final assembly.
SWIF manufacture in general, whether of the conventional or sidestepping type, is costly and the percentage yield of acceptable units is low. One cause of this low yield is found in transducer construction defects which are only detectable during the late stages of manufacture, that is, after the medium has been prepared and transducers have been formed. Consequently, such defects are found only after these expensive operations have been performed.
Accordingly, it is an object of the present invention to provide a novel method of manufacturing sidestepping SWIF devices.
Another object of this invention is improving the manufacturing yield of sidestepping SWIF's.
It is another object of the present invention to provide a novel sidestepping SWIF device.